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J . Phys. Chem. 1989, 93, 4890-4894
mM NaBr are shown in Figure 6. It can be seen that 0,:p steadily increases with time. This result can be explained either by a displacement of the probe toward the outer interface and/or by bromide penetration through the bilayer. Figure 7 shows data obtained for LN with a larger excess of bromide ions. In these experiments, total added electrolyte is 2 mM and 0.25 cm3 of (DODAC (2 mM) in 2 mM NaCI) are added to 2.25 cm3 of 2 mM NaBr. After 15 h, BB:P reaches a value of 0.87. These data can only be explained in terms of a steady penetration of bromide ions through the bilayer. Permeation of ions through simple tetraalkylammonium ionic vesicles has been extensively studied by Moss et a1.35-39 In particular, these authors found that dioctadecyldimethylammonium vesicles are remarkably efficient in avoiding anion permeation through the bilayer.39 For example, Ellman’s anion and a-iodosobenzoate can be separately entrapped in DODAC vesicles and maintained in the same aqueous solution for many hours with minimal reaction, even though they react within 10 s when free.3g Nevertheless, the data shown in Figures 6 and 7 imply a rather fast rearrangement of counterions in the asymmetric vesicle having different counterion composition at the outer and inner surfaces. The ability of a bilayer to avoid ion diffusion is determined by the packing of the alkyl chains, and permeability increases notably when this order is reduced, Le., when the vesicles are heated above the phase transition tempera t ~ r e , or~ when ~ ~ ’ divalent cations (i.e., Ca2+) are added which (35) Moss, R. A,; Hendrickson, T. F.; Swarup, S.; Hui, Y.; Marky, L.; Breslauer, K. I. Tetrahedron Lett. 1984, 25, 4063. (36) Moss, R. A,; Swarup, S.; Schreck, R. P. Tetrahedron Lert. 1985, 26, 603. (37) Moss, R. A,; Swarup, S.; Wilk, B.; Hendrickson, T. F. Tetrahedron Lett. 1985, 26, 4827. (38) Moss, R. A.; Schreck, R. P. J. Am. Chem. SOC.1985, 107, 6634. (39) Moss, R. A.; Swarup, S.;Zhang, H . J. Am. Chem. SOC.1988, 110, 2914.
can disturb the bilayer structure (for example, with didodecyl phosphate bilayer membra ne^.^' We consider that the most likely explanation of the increased permeation observed in the asymmetric bilayer is that changing the outer counterions imposes on the bilayer a tension that reduces the order and promotes rearrangement, most probably by enhancing the “flip-flop” movement, to achieve a thermodynamically stable state. Both effects, the reduced order of the bilayer and the increased flip-flop movement, can enhance the counterion permeation. Although, the driving force in this case is surely different, the results presented here provide another of a growing number of cases where the transit of impermeable of nearly impermeable ions across a bilayer is mediated by subtle changes in the external environment.4245
Acknowledgment. We are grateful to DICYT (Universidad de Santiago de Chile), DTI (Universidad de Chile), (Grant No. Q2085/8733), to FONDECYT (Grant No. 507/87), and to the U S . National Science Foundation (Grant No. C H E 86-16361) for financial support. We are also grateful to the U S . National Science Foundation for an award for International Cooperation (INT-8401143). E.A. is grateful to the John Simon Guggenheim Foundation for a Fellowship. Registry No. DODAC, 107-64-2; DODAB, 3700-67-2; LN, 2643828-8; D N A , 115034-83-3; Cl-, 16887-00-6; Br-, 24959-67-9. (40) Okahata, Y.; Lim, Han-Jin; Nakamura, Gen-ichi; Hachiya, S. J. Am. Chem. SOC.1983, 105, 4855. (41) Okahata, Y.; Lim, Han-Jin; Nakamura, Gen-ichi. Chem. Lett. 1983, 175. (42) Tabushi, I.; Kugimiya, S. J . Am. Chem. SOC.1985, 107, 1859. (43) Lee, L. Y.-C.; Hurst, J. K.; Politi, M.; Kurihara, K.; Fendler, J. H . J Am. Chem. SOC.1983. 105. 370. (44) Patterson, B. C.; Thompson, D. H.; Hurst, J. K. J. Am. Chem. SOC. 1988, 110, 3656. (45) Runquist, J. A.; Loach, P. A. Biochim. Biophys. Acta 1981,637,231.
Effects of Palladium Particle Size and Palladium Silicide Formation on Fourier Transform Infrared Spectra of CO Adsorbed on Pd/SiO, Catalysts Lien-Lung Sheu, Zbigniew Karpinksi,+and Wolfgang M. H. Sachtler* V. N . Ipatieff Laboratory, Center of Catalysis and Surface Science, Northwestern University, Evanston, Illinois 60208 (Received: September 7 , 1988; In Final Form: February 6, 1989)
Two major modes of CO adsorption on Si02-supported Pd reflect different extents of back-donation, which is, at least in part, controlled by the local electron density at the adsorption site. The fraction of CO in the bridging mode (B) increases and that of the linear mode (L) decreases, with increasing size of the Pd particles, indicating high electron density at Pd atoms in terraces of close-packed crystal faces, in agreement with Smoluchowski’s classical model.’ For samples reduced at 300 O C our data points and those of other authors are located on a common curve of B/L vs metal dispersion. Extensive reduction at 600 O C results in significantly lower B/L values, attributed to the incipient formation of a palladium silicide. Oxidation followed by reduction at 3 0 0 O C destroys the silicide, and the B/L value returns to the original curve.
Introduction Reduction of oxide-supported transition metals (e.g., Pt/A1203, Rh/Ti02, or Pd/Si02) under severe conditions (e.g., flowing hydrogen at 700 “C) often induces characteristic changes of the adsorptive and catalytic properties of the metal. Previous research has identified two major causes for these changes: 1. Metal particle growth, mainly by migration of primary particles over the support surface, and their coalescence.2 This results in a change of specific catalytic activity for structuresensitive reactions. On leave from Institute of Physical Chemistry, Polish Academy of Sciences, Warsaw, Poland. *Author to whom correspondence should be addressed. 0022-3654/89/2093-4890$01.50/0
2. Partial reduction of the supporting oxide, followed by chemical interaction with the metal. Well-documented examples are (a) formation of a PtAl alloy from Pt/A1203;3-5(b) formation of palladium silicide from Pd/Si02;6,7and (c) partial reduction ( 1 ) Smoluchowski, R. Phys. Reu. 1941, 60, 661.
(2) Ruckenstein, E.; Pulvermacher, B. AIChE J. 1973, 19, 356; J. Carol. 1913, 29, 224.
(3) Den Otter, G. J.; Dautzenberg, F. M. J . Coral. 1978, 53, 116 (4) Survs, J. W.; Mencik. Z. J. Catal. 1975, 40, 290. (5) Biker, R. T. K. In Metal-Support and Metal-Additive Eflects in Catalysis; Imelik, B., et al. Eds.; Elsevier: Amsterdam-Oxford-New York, 1982; p 37. (6) Juszczyk, W.; Karpinski, 2.;Pielaszek, J.; Ratajczykowa, I.; Stanasiuk, Z. Proceedings of the 9th International Congress on Catalysis (Calgary, June 27-July 1 , 1988); 1988; Vol. 3, p 1238.
0 1989 American Chemical Society
FTIR Spectra of CO Adsorbed on Pd/Si02 Catalysts of Ti02, followed by the formation of a TiO, overlayer on the metal particles, This phenomenon is often referred to as SMSI effect.+I I Much surface science work has been published to identify the SMSI effect of TiO,-supported metals after severe reduction. For palladium, Juszczyk and Karpinski7 recently identified Pd3Si by X-ray diffraction after reducing a physical mixture of Pd and Si02 in dry H 2 for 17 h at 600 “C. Some of these methods, however, are not applicable to supported metal catalysts with low metal loading. In particular for highly dispersed metals it is often difficult to decide whether an observed change in catalytic performance after reduction at elevated temperature is due to a chemical interaction with the support or to mere changes in metal particle size or shape. Particle growth and chemical interaction often occur simultaneously and contribute to changes in adsorption and catalysis. There is, hence, a need for a reliable detection method of incipient chemical interaction between metal and support. The present work addresses this problem by using the infrared (IR) spectra of chemisorbed C O on Pd/Si02 to probe both for changes in the Pd particle size and the incipient formation of palladium silicide upon reducing Pd/Si02. Two IR bands are characteristic for CO adsorbed on Pd/Si02, and it is known that the ratio of their intensities changes significantly when atoms such as Ag are interdispersed on the surface of the Pd particles.12 It is therefore expected that the formation of palladium silicides should reveal itself by a change of the band intensity ratio for their IR bands. The intensity of the IR bands of adsorbed CO also depends, however, on the geometry of the adsorbing Pd particle. Quantum mechanical calculations by Cederbaum et al.,13 Blyholder,I4 and Batra and Bagus15 show that bonding of CO on small metal clusters is essentially similar to the metal-CO interaction in metal carbonyls. This conclusion is confirmed by UPS data of FordI6 and Freske.17 The occurrence of different C-0 stretching IR bands was ascribed by Eischens and Pliskin18 to different adsorption modes, via., “linear” (or “terminal”) and “bridging” over two or more metal surface atoms. Later B l y h ~ l d e r ’ ~proposed **~ a different model, based on the c and dr-pr* scheme accepted for metal carbonyls. His more recent calculations2’ for CO adsorbed on Ni clusters reconcile both models: an increase in the number of metal atoms coordinated to the C O molecule results in a reduction of the C-O stretching frequency because the M-CO bond has more r c h a r a c t e r for higher coordination numbers due to the increased back-donation of metal electrons. Bradshaw and Hoffmann22studied CO on the Pd single crystal planes (loo), (1 1 l), and (210) by IR reflection-absorption; they conclude that C O adsorption is surface specific and discerns different localized sites at low coverage; Le., the description in terms of “linear” (L) and “bridging” (B) CO, as originally proposed by Eischens and Pliskin, is adequate. In the present paper results will be expressed in terms of the intensity ratio, B/L which provides internal normalization independent of the metal loading or wafer thickness. Present theory does not permit quantitative predictions how the B/L ratio should change with particle size or metal-support interactions. Qualitatively, it is expected that increasing isolation (7) Juszczyk, W.; Karpinski, Z., submitted for publication in J. Carol. (8) Tauter, S. J.; Fung, S. C.; Garten, R. L. J . Am. Chem. SOC.1978, ZOO, 170. (9) Sadeghi, H. R.; Henrich, V. E. J . Cutal. 1984, 87, 279. (IO) Takatani, S.; Chung, Y.-W. J . Catal. 1984, 90, 75. (11) Sanchez, M. G.; Gazquez, J. L. J . Card. 1987, 104, 120. (12) (a) Soma-Noto, Y.; Sachtler, W. M. H. J . C a r d 1987, 34, 162. (b) Primet, M.; Mathieu, M. V.; Sachtler, W. M. H. J . Catal. 1976, 44, 324. (13) Cederbaum, L. S.; Domcke, W.; von Niessen, W.; Brenig, W. 2. Phys. 1975, B21, 381. (14) Blyholder, G. J . Phys. Chem. 1975, 79, 756. (15) Batra, P.; Bagus, P. S. Solid State Commun. 1975, 16, 1097. (16) Ford, R. R. Adu. Catal. 1970, 21, 5 1 . (17) Freske, R. F. Prog. Inorg. Chem. 1976, 21, 170. (18) Eischens, R. P.; Pliskin, W . A. Adu. Catal. 1958, 10, 1. (19) Blyholder, G. J . Phys. Chem. 1964, 68, 2772. (20) Blyholder, G.;Allen, M. J . Am. Chem. SOC.1969, 91, 3158. (21) Blyholder, G. J . Vacuum Sci. Technol. 1974, 11, 865. (22) Bradshaw, A. W.; Hoffmann, F. M. Surf.Sci. 1978, 72, 513.
The Journal of Physical Chemistry, Vol. 93, No. 12, I989 4891 TABLE I: Catalyst Pretreatment and Metal Particle Size ~
~~
catalyst pretreatment 0.76 wt % Pd/SiO, “standard” H2. 300 “ C , 1 h, Ar, 300 OC, 1 h HTR H,,600 “C, 17 h, Ar, 600 “C, 1 h RG after H T R 02,300 O C , 0.5 h; H,, 300 OC, 1 h, Ar, 300 “ C , 1 h another condition H2, 430 “C, 12 h; Ar, 430 O C , 1 h, Ar, 2h
+
“standard” HTR
RG
1.58 wt % Pd/Si02 H2, 300 “C, 1 h, Ar, 300 “C, 1 h
H2, 600 O C , 17 h, Ar, 600 “ C ; evac, 600 OC, 1 h after H T R 02, 300 “ C , 0.5 h; H2, 300 “C, 1 h
+
CO/Pd: 0.62 0.34
0.34 0.52b
0.42 0.22 0.22c
‘Fractions exposed taken from ref 5. *In ref 6 the “equivalent” pretreatment was H,, 450 “C, 17 h; Ar, 500 OC,1 h. ‘Not measured, assumed to be the same as after H T R .
of adsorbing metal atoms will affect the bridging C O complex more adversely than the linear CO. The B/L ratio is, thus, expected to decrease with increasing metal dispersion (more comer atoms, less atoms on terraces), and it will also decrease when Pd atoms become isolated one from another by some chemical interaction with the support. This research attempts to check this expectation and to provide a basis for more quantitative predictions. As data on single crystals and other Pd samples show that the IR bands of CO adsorbed on Pd shift with coverage due to dipole-dipole coupling, direct repulsion, and indirectly through metal interaction, samples have to be compared to equal CO coverage. Our strategy is to expose each sample first to a CO pressure exceeding that required to complete monolayer coverage and then to purge all samples at the same temperature for the same length of time. The steep decrease of the heat of adsorption with coverage at 0 > 1, reported by Tracy and Palmbergz3 justifies the assumption that monolayer coverage is established by our procedure. To distinguish effects due to ordinary Pd particle growth from those caused by chemical interaction with the support (e.g., formation of palladium silicide at elevated temperature) we “regenerate” such samples. This procedure implies that samples that had been reduced at 600 “ C are first oxidized, and then reduced at 300 “C. In this procedure any palladium silicide is first converted to PdO + Si02and then to Pd + S O 2 , but large Pd particles will not be transformed into small Pd particles. Effects on the B/L ratio due to silicide formation are thus assumed to be eliminated by regeneration; no assumption has to be made concerning the average Pd particle size, which is measured by C O adsorption.
Experimental Section Catalysts. The preparation of 0.76 and 1.58 wt % Pd/Si02 catalysts has been described in an earlier paper.6 Davison 62 silica gel (100-120 mesh, acid washed) was impregnated by an incipient wetness technique with an aqueous solution of PdCI2. Fractions exposed after various pretreatments were determined by chemisorption of C O (D,o = CO/Pd) using a pulse technique (Table I in ref 6). The data are collected in Table I. For the “standard” reduction process, the Pd/Si02 was reduced at 300 OC under flowing H2with a flow rate of about 100 mL/min and then degassed at the same temperature for 1 h in flowing Ar. The high-temperature reduction (HTR) was carried out at 600 “ C for 17 h followed by degassing at the same temperature for 1 h. “Regeneration” of such HTR samples was done by first oxidizing at 350 “ C in flowing O2of 1 atm for 1 h, then reducing at 300 OC for 1 h, and degassing at 300 “C for 1 h. Regenerated samples are denoted as “RG”. Fourier Transformed IR Spectra of Adsorbed CO. Eighty milligrams of a Pd/Si02 catalyst was pressed into a disk wafer (23) Tracy, J. C.; Palmberg, P. W. J. Chem. Phys. 1969, 51, 4852.
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Sheu et al. perature reduction (HTR), and ”regeneration” (RG). It is clear that the B/L ratio of spectrum b after HTR (2.74) is lower than that of spectrum a after standard pretreatment (2.96) and markedly lower than that of c after “regeneration” (6.28).
?310
2190
7070
1950
1830
1710
1590
WAVENUhrBERkm- ’)
Figure 1. IR spectra of CO adsorbed on 0.76 wt % Pd/Si02 after pretreatments: “standard” (curve a), HTR (curve b), and “regeneration” (curve c ) .
with a 20-mm diameter. All catalyst pretreatments were performed in situ in a flow-type fused silica cell with NaCl windows. A detailed description of the FTIR cell is given in ref 24. Ultrahigh-purity H2 and Ar (both 99.999%, Matheson, IL, USA), further purified over molecular sieve and MnO/Si02 traps, have been used. Table I lists the sample pretreatments and the resulting metal particle sizes that were determined by CO adsorption. After each treatment the samples were cooled in Ar, and I R spectra were recorded with a Nicolet 60SX single-beam Fourier transform infrared spectrometer at a resolution of 1 cm-l (background spectrum). Generally, a scan number of 200 was chosen to improve the signal-to-noise ratio. C O (99.9%, Matheson, IL, USA), further purified over alumina, molecular sieve, and MnO/Si02 traps, was introduced onto the catalyst wafer at room temperature and under atmospheric pressure, and then the cell was purged with Ar at room temperature. The IR spectra of adsorbed CO were then obtained by the ratio of sample spectra to the background spectra and recorded as a function of purging time.
Results As gaseous and weakly adsorbed C O show up in the IR spectra, the Ar purging is essential for obtaining reproducible spectra. It was found that after purging in flowing Ar for 20 min, traces of gas-phase C O are still detected in our cell (as a “noise”) around 2100-2180 cm-I. After Ar purging for another 30 min, the I R spectrum shows that all gas-phase C O and a small part of the linearly bound adsorbed CO (at ca. 2080 cm-’) have disappeared, whereas the band at ca. 1980 c d , which is attributed to bridging CO, is unchanged. Upon further purging for 150 min, the ”linear” C O band markedly decreases and the “bridging” band diminishes slightly. In order to evaluate properly the effects of sample pretreatments, we shall report data obtained after a standardized purging procedure of 50 min in flowing Ar at 21 O C . The different pretreatment methods are enumerated in Table I. In Figure 1 the IR spectra of adsorbed CO on 0.76 wt % Pd/Si02 are displayed after the pretreatments: “standard reduction”, high-tem(24) Konishi, Y.;Ichikawa, M.;Sachtler, W. M. H. J . Phys. Chem. 1987, 91, 6286.
Discussion Effect of Pd Dispersion. The experimental data show that for our setup a 50-min purging time at room temperature is sufficient to remove all gaseous and weakly bonded CO. A comparison of the present data with those of other workers who have used similar precautions is, therefore, in order. The assignment of the IR bands of chemisorbed C O has been subject of some controversy. There is a universal agreement that they reflect the C-O stretching frequency, which becomes smaller when the bond between the carbon atom and the metal atoms becomes stronger. As mentioned in the Introduction, Blyholder showed that an important element determining the strength of the metal-carbon bond is the extent of back-donation of metal electrons to the **-orbital of adsorbed CO. What is still debated is which metal atoms will be stronger electron donors: (a) Pd atoms in terraces of Pd crystal surfaces; (b) protruding atoms, e.g., at corners and edges; or (c) Pd atoms in alloy surfaces, e.g., PdAg or PdSi. Sheppard and N g ~ y e nhave ~ ~ reviewed the relevant experimental arguments, and they conclude that the band assignment originally proposed by Eischens and Pliskin is correct; Le., as the lower CO stretching frequencies are due to bridging CO, the higher frequency has to be assigned to the linear CO bonded to one metal atom. The data by Soma-Noto and SachtlerI2 show that isolation of Pd atoms in PdAg surfaces strongly reduces the relative intensity of the low-frequency band. Bradshaw and Hoffmann22conclude that the *-character of the M-CO bond is higher when the coordination number is larger. All these data suggest that backdonation from the metal to the ligand is highest for Pd atoms in terraces of Pd crystal surfaces. Isolated or protruding atoms (e.g., in corner or edge positions or in “open” crystal faces) appear to be less efficient electron donors. The theoretical concept that protruding atoms or atoms in open crystal faces have lower electron densities than atoms in closepacked faces was first introduced in 1941 by SmoluchowskL1 He showed that the density of metal electrons is “smoothened” near the surface of a metal crystal. For crystal faces with widely spaced atom arrangements, electrons spill over into the valleys between the surface atoms, leaving the electron cloud above each atom electron deficient. This model has succeeded in rationalizing the large anisotropy of the electronic work function of metals and is confirmed by experimental data of single crystals, including field emission data. When Smoluchowski’s theory is applied to chemisorption of CO, one would expect that back-donation from the metal will be lower for protruding atoms, e.g., at corners, than for atoms in terraces of Pd crystal surfaces. The linear mode of adsorbed CO with high stretching frequency is then expected to prevail on protruding atoms where back-donation is small; the bridging complex with low-frequency bands will preferentially be located on terraces. This conclusion agrees with the experimental facts: the linear mode of adsorbed C O prevails on single-crystal faces, as the data of Bradshaw and Hoffmann confirm; but in carbonyl cluster complexes of group VI11 metals, such as Rh, Ru, Ni, and Ir, the majority of the CO ligands is of the terminal type.26 The conclusion that linear CO is preferentially formed on low-coordinated Pd atoms immediately implies that the B/L ratio should decrease with increasing Pd particle size, because smaller particles expose a larger fraction of low-coordinated (corner or edge) atoms than larger particles, if all other factors are kept equal. Indeed, van Hardeveld et showed that a direct relation exists between the IR spectra of adsorbed C O and the Pd particle size. This conclusion is confirmed and amplified by the present results. ~
~
( 2 5 ) Sheppard, N., Nguyen, T T Adu. Infrared Raman Spectrosc. 1978, 5 , 67. (26) Braunstein, P.;Rose, J. In Stereochemistry of Organometallic and
Inorganic Compounds; Bernal, I., Ed.; Elsevier: Amsterdam, 1988: Vol. 3 (27) Van Hardeveld, R.; Hartog, F. Adu. Catal. 1972, 22, 7 5 .
FTIR Spectra of C O Adsorbed on Pd/Si02 Catalysts 1
11 I
The Journal of Physical Chemistry, Vol. 93, No. 12, 1989 4893 10
A
10 -
9-
t
0 -
7 -
6-
54 -
3-
2-
0.00
0.20
0.40
0.60
0.80
1.00
Dispersion of Pd Particles Figure 2. Effect of Pd dispersion (in Pd/Si02 catalysts) on B/L ratio. (a) standard 0.76 wt % Pd/Si02, (b) HTR 0.76 wt % Pd/Si02, (c) RG 0.76 wt % Pd/Si02, (d) standard 1.58 wt % Pd/Si02, (e) HTR 1.58 wt % Pd/Si02, and (f) RG 1.58 wt % Pd/Si02. ( 0 )this work, Pd; (+) this work, Pd silicides; (v)Bell et (m) Boudart et al.;28929 (A)Palazov et
In Figure 2, the B/L ratio is shown as a function of the particle size. All data of samples that were reduced under standard conditions are located on the same curve, showing that the B/L ratio increases with particle size in accordance with the geometric model. For a meaningful comparison with results of other authors, we have chosen conditions that were likely to establish the same coverage of the surface with either CO species as for the data reported in the literature. In Figure 2 our data, and those reported by three groups are combined, viz., Ichikawa et al.,28929Hicks et al.,30and Palazov et al.31 For obvious reasons, we only used those IR spectra of adsorbed C O that were recorded in virtual absence of C O in the gas phase, Le., either after evacuation at room temperature for 30 min28v29(or 15 min, ref 3 1) or after controlled dosing of CO to get virtually complete surface coverage.N Palazov et aL3] report a B/L value of 85/15 for Pd/Si02, with a fraction of surface exposed Pd of 0.35. With the other literature data cited we integrated the areas below the respective peaks to calculate the B/L ratios. As the curve in Figure 2 shows all these results, including our own data, show an excellent fit on one common curve. We, therefore, conclude that the B/L ratio of adsorbed C O on Pd/Si02, after reduction and evacuation under the conditions defined above, is defined by the Pd particle size. This conclusion suggests two consequences: (1) the B/L ratio may serve as a convenient experimental method to estimate metal dispersion; (2) the result lends some credence to the model, Le., lower local electron density and lower back-donation to adsorbed C O by protruding or isolated Pd atoms than by Pd atoms in terraces of close-packed crystal faces. The B/L ratios of the band intensities may differ from the actual concentration ratios of the corresponding species on the surface, because it is possible that the extinction coefficients for the bands of linear and bridging C O are different. For most polynuclear metal carbonyls of known stoichiometry, very similar extinction coefficients are quoted for terminal and bridging C O ligands, but no polynuclear carbonyls seem to be known for palladium. For CO on supported palladium literature data of (28) Ichikawa, S.;Poppa, H.; Boudart, M. J. Catal. 1985, 91, 1. (29) Ichikawa, S.;Poppa, H.; Boudart, M. In Catalytic Maferials: Relationship between structure and Reactivity; Whyte, T. E., et al., Eds.; ACS Symposium Series 248; American Chemical Society: Washington, DC, 1984; p 439. (30) Hicks, R. F.; Yen, Q.-J.; Bell, A. T. J . Catal. 1984, 89, 498. (31) Palazov, A; Chang, C. C.; Kokes, R. J. J . Catal. 1975, 36, 338.
1 0.1
1
Dispersion of Pd Particledlog) Figure 3. Plot of log of B/L ratio vs log of Pd dispersion.
extinction coefficients are conflicting; attempts to calculate them from spectroscopic data require assumptions regarding the adsorption stoichiometry of the bridging CO. Some authors assume COb/Pd, = 1/2; others believe that two bridging C O can share one Pd atom, Le., CO,/Pd, = 1. For the present purpose a knowledge of these extinction coefficients is not required; it suffices to correlate the measured B/L values with the Pd dispersion, conventionally defined by the molar ratio of adsorbed C O and Pd . In Figure 3 the experimental data are presented in a doublelogarithmic plot. A straight line is found with a slope of -1. This result is reminiscent of the scaling law, recently proposed by Farin and A ~ n i for r ~the ~ structure sensitivity of catalytic reactions. In the case of supported particles of given shape but different sizes, defined by the linear parameter R (for cubes, R = length of an edge), the following geometric relations hold: number of surface atoms inside planes a R2; number of surface atoms in edge positions a R’; dispersion = surface atoms/total atoms a R1. If the IR intensity of the L band was simply proportional to the number of edge atoms and that of the B band to the number of atoms in planes, the B/L ratio would be proportional to R and hence to (dispersion)-’, as is indeed found to be the case in Figure 3. We certainly do not wish to stretch the simple geometrical model beyond the limits that are justified by the assumptions and the experimental data. There is no doubt that linear CO does also exist on terraces of single crystals, and bridging CO is present in cluster carbonyls that do not have any terraces. It appears, however, that the data justify the conclusion that linear C O is somewhat more stable on atoms of low coordination number, while bridging C O has an energetic preference for highly coordinated Pd atoms (e.g., in terraces of stable crystal faces, where backdonation is large). Effects of Chemical Interaction with the Support. A very significant departure from the curve in Figure 2, defined by all samples reduced under standard conditions, is observed for samples that have been reduced at much higher temperatures. It is very relevant that, after the “regeneration” procedure, the data are, again, located on the common curve of all samples reduced under standard conditions. In the Introduction it was mentioned that independent data prove the formation of Pd silicide upon high-temperature reduction of Pd/Si02,6 while other data show that dilution of Pd with atoms of another element, unable to form strong bonds with CO, induces a pronounced decrease in the B/L ratio.I2 Therefore, it appears that the IR spectrum of adsorbed C O is another probe for detecting metal-support interactions in case of Pd/SiOz. (32) Farin, D.;Avnir, D. J . Am. Chem. SOC.1988, 110, 2039.
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The present results thus show that isolation of surface Pd atoms can be increased in two ways: (1) decreasing the particle size, Le., producing a high fraction of coordinatively unsaturated Pd atoms; (2) interdispersing foreign atoms in the Pd surface, e.g., forming PdAg alloys or PdSi,. Compound formation is found to be a much more efficient way to isolate Pd atoms. This is in line “ i t h Blyholder’s quantum mechanism calculations;21the extent of back-donation is reduced more strongly by engaging a Pd atom into bonding to an adjacent Si atom than by merely changing its coordination to other Pd atoms. It is important that IR spectroscopy permits detection of Pd silicide formation in an incipient stage where this cannot be easily registered by other instruments. It is also interesting that the departure of the B/L ratio from the “standard” curve is smaller for the sample with high Pd loading. This may be due to surface
enrichment of the surface with Pd. Since Pd, unlike Si, is able to form strong chemical bonds with CO, the surface should become enriched in Pd during exposition to C O because of the usual “chemisorption induced surface segregation” which was proven for numerous alloy systems.)) This surface enrichment of PdSi, in Pd should be more pronounced for particles with larger volume than for small particles for which a large atomic fraction is located at the surface. It is also conceivable that the extent of the interaction between Pd and silica depends on the particle size of the Pd. At present we have no additional data for testing these speculations. Registry No. Pd, 7440-05-3; CO, 630-08-0. (33) (a) Bouwman, R.; Sachtler, W. M. H. J. Caral. 1970, 19, 127. (b) Bouwman, R.; Lippits, G. T. M.; Sachtler, W. M. H. J . Card. 1972, 25, 350. (c) Sachtler, W. M. H. Vide 1973, 163, 19.
Photorheological Effects in Micellar Solutions Containing Anthracene Derivatives. A Rheological and Statlc Low Angle Light Scatterlng Study Thomas Wolff,* Claus-Stephan Emming, Thomas A. Suck, and Gunther von Bunau Institut fur Physikalische Chemie, Uniuersitat Siegen, 0 - 5 9 0 0 Siegen, West Germany (Received: September 21, 1988; In Final Form: December 16, 1988/
Viscosities, flow behavior, and masses of micelles in aqueous solutions of cetyltrimethylammonium bromide (CTAB) were determined in the presence of 9-substituted anthracenes. Greatly enhanced viscosities were found when small amounts of nonpolar anthracene derivatives (methyl, ethyl, n-propyl, n-butyl, n-pentyl) are added to pure CTAB solutions while strictly Newtonian flow was observed. Micellar masses exceeding that of pure CTAB micelles by factors of 35-260 were determined. Upon photochemical conversion of 95% of the anthracenes to dimers, viscosities dropped but no significant changes of micellar masses took place. In solutions of CTAB and added 9-anthracenecarboxylic acid non-Newtonian flow behavior, such as rheopexy, viscoelasticity, and thixotropy, was observed being influenced by photodimerization. The rheopectic flow behavior is shown to be connected with low micellar masses
Introduction Flow behavior and viscosity of aqueous micellar solutions are drastically altered by solubilizing but small amounts of suitable aromatic substances that may be called “rheologically active”. Other aromatic substances are “rheologically inactive” and have little influence on the flow properties of these solutions. In previous work it was shown that certain aromatic compounds of the one class may be photochemically transformed into products belonging to the Accordingly these processes are accompanied by “photorheological effects”, such as viscosity changes, transformations of Newtonian into non-Newtonian fluids, and even transitions between different lyotropic liquid crystalline phases.2 Examples are cis-trans isomerizations of some stilbene derivatives) and photodimerizations of some 9-substituted anthracene derivatives in aqueous micellar solutions of the cationic tenside cetyltrimethylammonium bromide (CTAB) and the nonionic tenside Triton X-100.5 It has been previously suggested that the macroscopically observable viscosity effects are due to microscopic changes in size and/or shape of micellar aggregates specifically induced by the (1) Milller, N.; Wolff, T.; von Bilnau, G. J. Photochem. 1984, 24, 37. (2) Wolff, T.; von Bilnau, G. Eer. Bunsen-Ges. Phys. Chem. 1984, 88,
1098. (3) Wolff, T.; von Biinau, G. J. Photochem. 1986, 35, 239. (4) Wolff, T.; Suck, T. A,; Emming, C.-S.; von Bilnau, G. Prog. Colloid Polym. Sci. 1987, 73, 18. ( 5 ) Wolff. T.: Schmidt, F.; von Biinau, G. J. Photochem. Photobiol., A, in press.
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respective solubilizates. In order to test this assumption directly, we have carried out parallel experiments to obtain macroscopic as well as microscopic data of the same aqueous micellar CTAB solutions before and after photochemical transformation of rheologically active additives solubilized in these systems. The macroscopic flow behavior was examined with a computer-controlled rotating viscometer that discriminates between Newtonian and nowNewtonian fluids. Microscopic information on the size of the micellar aggregates before and after photoconversion was obtained from static low angle scattering of these solutions. Newtonian flow is exhibited by aqueous CTAB solutions containing small amounts of nonpolar 9-substituted anthracenes, Le., 9-methyl-, 9-ethyl-, 9-n-propyl-, 9-n-butyl-, and 9-n-pentylanthracene, all of which are rheologically active inasmuch as their solubilization leads to a drastic increase of the solution visco~ity.~*~ Upon irradiation of these solutions photodimers are formed according to
During photochemical conversion the solution viscosity changes in a specific and complicated way. A particularly large viscosity decrease was found in the cases of n-butyl- and n-pcntylanthracenes which allow an almost complete (>90%) photoconversion without the photodimers precipitating from the micellar 0 1989 American Chemical Societv
